Trailer Control ValveThe independent trailer control valve (fig. 7-48)provides the operator with control of the trailing load atall times. This valve functions in the same manner asthe treadle valve except that the handle is turned, ratherthan depressed, to operate the valve.Quick-Release ValveThe quick-release valve (fig. 7-49) exhausts brakechamber air pressure and speeds up brake release byreducing the distance the air would have to travel backto the brake valve exhaust port.When the brakes are engaged, air from the brakevalve enters into the quick-release valve, forcing thediaphragm down and closing off the exhaust port. Thisaction allows air pressure to rush through thequick-release valve outlet ports to the wheel brakechambers. When the brakes are released, the airpressure above the quick-release diaphragm isFigure 7-48.Trailer control valve.Figure 7-49.Quick-release valve.exhausted at the brake valve. As air pressure above thediaphragm is released, the air pressure below thediaphragm raises off the exhaust port. This actionallows the air in the brake chambers to exhaust at thequick-release valve.When air is leaking from the system, a leakage testcan determine if there is air leaking at the quick-releasevalve. The leakage test is performed with the brakesapplied and coating the exhaust port with soapsuds. Ifair bubbles form, this is a sign of a defective valve,which can be corrected by either cleaning andreplacing worn parts or by replacing the unit. Dirt,worn diaphragm, or a worn seat causes leakage.Combined-Limiting and Quick-Release ValveThe combined-limiting and quick-release valve(fig. 7-50) is used in combination with a two-waycheck valve in the air brake system of trucks andtractors. The combined-limiting and quick-releasevalve is interchangeable in mounting with thequick-release valve and serves the same purpose withthe additional function of providing an automaticreduction of front-wheel brake pressure, at the optionof the operator, on slippery roads.Tractor Protection ValveThe primary purpose of the tractor protectionvalve (fig. 7-51) is to protect the tractor air brakesystem under trailer breakaway conditions and underconditions where severe leakage develops in thetractor or trailer.Figure 7-50.Combined-limiting and quick-release valve.7-40